The frequency causes the fringing pattern to change

The frequency causes the fringing pattern to
change due to change in frequency,
The RLG determines the angle by counting
pulses from the photodetector.
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Inertial Navigation – Gyroscopes
Ring Laser Gyro
Problems:
•Lock-in at low rotation rates due to weak coupling between
the two resonant systems (coupling due to mirror backscatter)
Analagous to static friction (stiction) in mechanical
systems
Causes a dead zone
Alleviated by “dithering” the gyro at a few hundred Hz
•Random loss of pulses at the output ( causes “drift”)

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RLG
Note:
The sensitivity (resolution) of Ring Laser Gyro is:
N=4A/λL
Where: N is the number of fringes per radian
A is the area enclosed by the path
L is the Length of the path
λ is the wave length of the light
Note that the larger the area, the more sensitive the gyro
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Example:
RLG with an equilateral triangular path of length 21cm
and wavelength 980nm
- sides are 7 cm each
- A=(b*h)/2 =3.5*7*.866 =21.2cm
2
- N=4A/λL = 4*21.2x10 -4 /(980x10 -9 x21x10 -2 )=41205
Fringes per radian or 719 fringes per degree
Approximately 0.0014 degrees per fringe
RLG

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Inertial Navigation – Gyroscopes
Fibre Optic Gyro
Similar concept to RLG except that amplification is not used
Two strands of optical fibre are wound in opposite directions on a
coil
Laser light is sent from a single source down both fibres
The outputs of the two fibres are combined at a photodiode
Rotation of the coil around its axis causes the two paths to have
different lengths and the output of the photodiode provides a light
dark pattern. Each cycle indicates an increment of angular
rotation
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Inertial Navigation – Gyroscopes
Fibre Optic Gyro
Has the advantage of being rugged and relatively cheap
Sensitivity increases with length of fibre
Unfortunately, the longer the fibre, the lower the output signal.
Used on low performance systems

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Inertial Navigation – Gyroscopes
MEMS Gyro
All gyros to date have been quite large
in fact the sensitivity of spinning mass gyros and RLGs are a
direct function of their size.
Efforts are being made to apply MEMS technology to gyros as
well as to accelerometers
MEMS gyros exhibit high drift rates and thus are not suitable for
commercial aviation use
They are used in conjunction with GPS in “coupled” systems
which use the best characteristics of each
MEMS gyro:
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Inertial Navigation – Strapdown Systems
The main problem for an INS is to separate the
vehicle acceleration from the effect of gravity on the
accelerometers
In the stable platform, this is done by maintaining the
accelerometers perpendicular to the gravity vector
which allows us to ignore the effect of gravity
Another approach is to keep track of the gravity
vector and subtract its effect from the outputs of the
accelerometers
This is an analytical or computational
implementation. Made feasible with advancements in
microprocessors